Engines operating with one or more cylinders as dedicated exhaust gas recirculation (EGR) cylinders enjoy greatly simplified controls and pressure management, fewer hardware devices, and other benefits. However, these simplifications come at the cost of a loss of control over the system, including a loss of control of the EGR fraction during low load and transient conditions. When nominal cylinders are dedicated to providing EGR, and standard fueling and controls are applied, the EGR fraction provided by the cylinders is limited to the simple ratio of the number of EGR cylinders to the total number of cylinders. For example, an engine with one cylinder dedicated to EGR and four cylinders total will operate at a 25% EGR fraction if all of the cylinders are operated in the same manner.
However, during transient conditions the EGR fraction due to volume filling dynamics in the dedicated EGR flow path can vary significantly from the EGR fraction. In addition, in certain steady state conditions, the volumetric efficiency change with load can cause deviations of the EGR fraction from its expected value. When the EGR fraction decreases from its expected value, knock conditions can be developed. When the EGR fraction increases from its expected value, cylinder misfire and combustion instability can result. Therefore, further technological developments are desirable in this area.